It is known that the flame resistance of synthetic resins, in particular polyurethanes resins, can be increased by the addition of unreactive low molecular weight phosphoric or phosphonic acid esters during the manufacturing process. This procedure is, however, limited by the fact that if the desired mechanical properties are to be obtained, these low molecular weight compounds may only be used in such limited quantities that they are insufficient to ensure complete flame resistance. The procedure is also limited by the fact that these additives tend to migrate from the resin, because of their low molecular weight.
Attempts have been made to overcome this difficulty by incorporating halogen containing polycarboxylic acids or polyhydroxyl compounds into the molecular structure. Such halogenated components include tetrachlorophthalic acid, dibromophthalic acid or hexachloroendomethylene tetrahydrophthalic acid. Polyesters produced from such components have a much improved flame resistance (e.g. after they have been foamed with polyisocyanates), but such resistance is still insufficient in many cases. Other disadvantages lie in the fact that these polyesters are difficult to mix with polyisocyanates at room temperature because of their high viscosity, so that processing difficulties arise during the production of foams. Moreover, these polyesters tend to give rise to brittle foams when reacted with polyisocyanates so that they can only be converted into foams of satisfactory mechanical quality if they are mixed with the usual polyesters. In that case, however, the flame resistance achieved is partly lost. Furthermore, many of the conventional halogen containing flame retarding agents liberate corrosive gases such as hydrogen chloride or hydrogen bromide on combustion.
Flame resistant polyurethane resins which have good mechanical properties are obtained when using polyisocyanates which contain phosphoric acid or thiophosphoric acid groups (for example, the p-isocyanatophenyltriester of phosphoric acid). Phosphoric ester triisocyanates, however, can only be obtained by multistage processes and their use is therefore often uneconomical.
Hydrocarbon phosphonyl diisocyanates have also been used for the production of flame resistant foams. These diisocyanates, however, are acyl isocyanates, which are not only physiologically unpleasant because of their odor and vapor pressure but also because they are excessively reactive and readily saponified. Satisfactory foams, then, can only be obtained using usch isocyanates if the isocyanates are mixed with considerable quantities of the usual polyisocyanates such as tolylene diisocyanate. It is obvious, however, that the flame retarding properties are then lost.
The use of phosphorus containing polyether and polyester polyols for the production of polyurethane foams is also known in the art. These products, however, give rise to copious production of fumes when subjected to heat. Moreover, they are in many cases difficult to handle because of their viscosity which may interfere with the foaming process.